LibWeb: Store box's containing line box fragment in FormattingState

Layout should not change any properties of a box until the moment a
FormattingState is committed.

Userland/Libraries/LibWeb/Layout/Box.cpp

@@ -236,7 +236,7 @@ const Gfx::FloatRect Box::absolute_rect() const
     return rect;
 }
 
-void Box::set_containing_line_box_fragment(LineBoxFragmentCoordinate fragment_coordinate)
+void Box::set_containing_line_box_fragment(Optional<LineBoxFragmentCoordinate> fragment_coordinate)
 {
     m_containing_line_box_fragment = fragment_coordinate;
 }

Userland/Libraries/LibWeb/Layout/Box.h

@@ -14,6 +14,11 @@
 
 namespace Web::Layout {
 
+struct LineBoxFragmentCoordinate {
+    size_t line_box_index { 0 };
+    size_t fragment_index { 0 };
+};
+
 class Box : public NodeWithStyleAndBoxModelMetrics {
 public:
     struct OverflowData {
@@ -82,11 +87,7 @@ public:
 
     bool is_body() const;
 
-    struct LineBoxFragmentCoordinate {
-        size_t line_box_index { 0 };
-        size_t fragment_index { 0 };
-    };
-    void set_containing_line_box_fragment(LineBoxFragmentCoordinate);
+    void set_containing_line_box_fragment(Optional<LineBoxFragmentCoordinate>);
 
     StackingContext* stacking_context() { return m_stacking_context; }
     const StackingContext* stacking_context() const { return m_stacking_context; }

Userland/Libraries/LibWeb/Layout/FormattingState.cpp

@@ -44,6 +44,7 @@ void FormattingState::commit()
             box.set_offset(node_state.offset);
             box.set_content_size(node_state.content_width, node_state.content_height);
             box.set_overflow_data(move(node_state.overflow_data));
+            box.set_containing_line_box_fragment(node_state.containing_line_box_fragment);
         }
 
         // For block containers, transfer line boxes.

Userland/Libraries/LibWeb/Layout/FormattingState.h

@@ -63,6 +63,8 @@ struct FormattingState {
             return *overflow_data;
         }
 
+        Optional<LineBoxFragmentCoordinate> containing_line_box_fragment;
+
         // NOTE: NodeState is ref-counted and accessed via copy-on-write helpers below.
         size_t ref_count { 1 };
         void ref()

Userland/Libraries/LibWeb/Layout/LineBuilder.cpp

@@ -51,16 +51,15 @@ LineBox& LineBuilder::ensure_last_line_box()
 
 void LineBuilder::append_box(Box const& box, float leading_size, float trailing_size)
 {
-    auto const& box_state = m_formatting_state.get(box);
+    auto& box_state = m_formatting_state.get_mutable(box);
     auto& line_box = ensure_last_line_box();
     line_box.add_fragment(box, 0, 0, leading_size, trailing_size, box_state.content_width, box_state.content_height, box_state.border_box_top(), box_state.border_box_bottom());
     m_max_height_on_current_line = max(m_max_height_on_current_line, box_state.content_height);
 
-    // FIXME: Move this to FormattingContext!
-    const_cast<Box&>(box).set_containing_line_box_fragment({
+    box_state.containing_line_box_fragment = LineBoxFragmentCoordinate {
         .line_box_index = m_containing_block_state.line_boxes.size() - 1,
         .fragment_index = line_box.fragments().size() - 1,
-    });
+    };
 }
 
 void LineBuilder::append_text_chunk(TextNode const& text_node, size_t offset_in_node, size_t length_in_node, float leading_size, float trailing_size, float content_width, float content_height)